Phase shift keying of digital signals has been used for low power signal transmission, and has been found to be more efficient than frequency shift keying. Coherent phase shift keying has been used but has the disadvantage that it is necessary to derive an accurate phase reference signal for use in the detection process at the receiver. To avoid this, differential phase shift keying, wherein the information is transmitted by the changes in phase, has been used, and this has been found to have the advantage that it is more tolerant of channel variations.
In prior differential phase shift systems, the detection circuits have used delay lines or high Q, accurately tuned circuits for storing one bit of the signal to compare the next bit therewith. These circuits are quite critical and costly. Further, delay lines and high Q, precision tuned circuits cannot be implemented in integrated circuit form, and cannot be provided as small inexpensive items as required for many applications, such as paging receivers. Prior detection circuits also consumed substantial power, which is particularly objectionable in portable devices.